1. Field of the Invention
The present invention relates to a high-frequency tuning circuit and, more particularly, to a double-tuned circuit of a television tuner that can be switched so as to permit tuning to multiple frequency bands.
2. Description of the Related Art
A conventional double-tuned circuit of a tuner will be described with reference to FIG. 10 through FIG. 12 and FIG. 4. Referring to FIG. 10, the double-tuned circuit is constructed by a primary tuning circuit 51 and a secondary tuning circuit 52. The primary tuning circuit 51 is constructed by a dc-blocking capacitor 53 and a varactor diode 54 connected in series, which are connected in parallel with a high-band receiving tuning coil 55, a low-band receiving tuning coil 56, a resistor 57, a coupling coil 58, and a dc-blocking capacitor 59 connected in series in the order shown in the drawing. The anode of the varactor diode 54 is grounded, and the cathode thereof is connected to the dc-blocking capacitor 53. The other end of the dc-blocking capacitor 59 is also grounded. The connection point of the dc-blocking capacitor 53 and the tuning coil 55 provides an input end of the double-tuned circuit 51, and is connected to a high-frequency amplifier 60 in the preceding stage.
Furthermore, a dc-blocking capacitor 61, a switching diode 62, and a dc-blocking capacitor 63 that are connected in series are provided between the connection point of the tuning coil 55 and the tuning coil 56, and the ground. The anode of the switching diode 62 is connected to the dc-blocking capacitor 61 and the cathode thereof is connected to the dc-blocking capacitor 63.
The connection point of the dc-blocking capacitor 61 and the switching diode 62 is connected to a high-band receiving changeover terminal 65 via a feed resistor 64.
The connection point of the switching diode 62 and the dc-blocking capacitor 63 is connected to a low-band receiving changeover terminal 67 via a feed resistor 66.
A bias resistor 68 is provided between the connection point of the switching diode 62 and the dc-blocking capacitor 63, and the ground.
The connection point of the dc-blocking capacitor 53 and the varactor diode 54 is connected to a tuning voltage terminal 70 via a feed resistor 69.
Furthermore, the secondary tuning circuit 52 is constructed by a varactor diode 71 connected in parallel to a high-band receiving tuning coil 72, a low-band receiving tuning coil 73, a resistor 74, a dc-blocking capacitor 75, the coupling ciol 58, and the dc-blocking capacitor 59 that are connected in series in the order shown in the diagram. The anode of the varactor diode 71 is grounded, and the cathode thereof is connected to the tuning coil 72. A varactor diode 76 and a dc-blocking capacitor 77 connected in series are connected to the connection point of the varactor diode 71 and the tuning coil 72. The anode of the varactor diode 76 is connected to the dc-blocking capacitor 77, and the cathode thereof is connected to the tuning coil 72. The other end of the dc-blocking capacitor 77 provides an output end of the double-tuned circuit, and is connected to a mixer 78 in the succeeding stage. The mixer 78 receives an oscillation signal from an oscillator (not shown), and outputs an intermediate frequency signal.
A dc-blocking capacitor 79 and a switching diode 80 connected in series are provided between the connection point of the tuning coil 72 and the tuning coil 73 and the connection point of the switching diode 62 and the dc-blocking capacitor 63. The anode of the switching diode 80 is connected to the dc-blocking capacitor 79, and the cathode thereof is connected to the dc-blocking capacitor 63.
The connection point of the dc-blocking capacitor 79 and the switching diode 80 is connected to the high-band receiving changeover terminal 65 via a feed resistor 81.
The connection point of the switching diode 80 and the dc-blocking capacitor 63 is connected to the low-band receiving changeover terminal 67 via the feed resistor 66.
The connection point of the varactor diode 71 and the tuning coil 72 is connected to the tuning voltage terminal 70 via a feed resistor 82.
In the configuration set forth above, a voltage is applied to the high-band receiving changeover terminal 65 or the low-band receiving changeover terminal 67 to cause the switching diode 62 and the switching diode 80 to turn ON or OFF thereby to switch the double-tuned circuit between a high-band receiving mode and a low-band receiving mode.
To switch the double-tuned circuit of the tuner shown in FIG. 10 to the mode for receiving a high-band television signal (e.g. 170 MHz to 222 MHz), a voltage of 5V, for example, is applied to the high-band receiving changeover terminal 65, while no voltage is applied to the low-band receiving changeover terminal 67. This causes a forward voltage to be applied to the switching diode 62 and the switching diode 80, and both switching diodes 62 and 80 turn ON. This in turn causes the connection point of the high-band receiving tuning coil 55 and the low-band receiving tuning coil 56 to be grounded, and also causes the connection point of the high-hand receiving tuning coil 72 and the low-band receiving tuning coil 73 to be grounded. As a result, the varactor diode 54 and the high-band receiving tuning coil 55 are interconnected in parallel in the primary tuning circuit 51, and the varactor diode 71 and the high-band receiving tuning coil 72 are interconnected in parallel in the secondary tuning circuit 52. A high-frequency equivalent circuit at that time will be the double-tuned circuit shown in FIG. 11 when the dc-blocking capacitor and the resistor are ignored, thus allowing a desired tuning frequency to be obtained by regulating the voltage applied to the varactor diodes 54 and 71.
To switch the double-tuned circuit of the tuner shown in FIG. 10 to the mode for receiving a low-band television signal (e.g. 90 MHz to 108 MHz), a voltage of 5V, for example, is applied to the low-band receiving changeover terminal 67, while no voltage is applied to the high-band receiving changeover terminal 65. This causes a reverse voltage to be applied to the switching diode 62 and the switching diode 80, and both switching diodes 62 and 80 turn OFF. As a result, the primary tuning circuit 51 turns into a parallel tuning circuit (hereinafter referred to as xe2x80x9cthe main tuning circuitxe2x80x9d) composed of the high-band receiving tuning coil 55, the low-band receiving tuning coil 56, the coupling coil 58, and the varactor diode 54. The secondary tuning circuit 52 turns into a parallel tuning circuit composed of the high-band receiving tuning coil 72, the low-band receiving tuning coil 73, the coupling coil 58, and the varactor diode 71. Thus, a desired tuning frequency can be obtained by regulating the voltage applied to the varactor diodes 54 and 71.
In the double-tuned circuit switched to the low-band receiving mode, a reverse voltage is being applied to the switching diodes 62 and 80. In general, when a reverse voltage is applied to a diode, an inter-terminal capacitance of, for example, approximately 0.2 pF is generated. The resistors 64 and 81 also have some inter-terminal capacitance. If the inter-terminal capacitance of the switching diodes 62 and 80 under the reverse voltage and the inter-terminal capacitance of the resistors 64 and 81 become no longer negligible, then the high-frequency equivalent circuit in the double-tuned circuit in the low-band receiving mode will be the double-tuned circuit shown in FIG. 12.
In the example described above, the capacitors 83, 84, 85, and 86 are respectively equivalent to the inter-terminal capacitance of the switching diode 62, the switching diode 80, the resistor 64, and the resistor 81.
Referring to FIG. 12, in the primary tuning circuit 51, the varactor diode 54, the tuning coil 55, and the capacitors 83 and 85 make up an additional tuning circuit 87 (hereinafter referred to as the xe2x80x9cparasitic tuning circuitxe2x80x9d) separately from the main tuning circuit. Likewise, in the secondary tuning circuit 52, the varactor diode 71, the tuning coil 72, and the capacitors 84 and 86 make up a parasitic tuning circuit 88. The primary parasitic tuning circuit 87 and the secondary parasitic tuning circuit 88 share virtually the same tuning frequency. When, for example, the desired tuning frequency in the main tuning circuit is 127 MHz, the tuning frequency in the parasitic tuning circuits 87 and 88 appears in a UHF band in the range of 600 to 700 MHz.
Thus, the double-tuned circuit in the low-band receiving mode will have, for example, a frequency selectivity indicated by the solid line in FIG. 4. The frequency selectivity shows a peak owing to the tuning frequency (indicated by B in FIG. 4) of the parasitic tuning circuits 87 and 88, appearing separately from the tuning frequency (indicated by A in FIG. 4) of the main tuning circuit. The prior art has been presenting the following problem. If a signal in the tuning frequency range of the parasitic tuning circuits 87 and 88 is applied to the mixer 78 connected to the output end of the double-tuned circuit and mixed with an oscillation signal by the mixer 78, then the signal generated based on a sum of or a difference between an N-multiple of the frequency of an oscillation signal and the frequency of a signal at a tuning frequency of the parasitic tuning circuits 87 and 88 interferes with an intermediate frequency signal (54 MHz to 60 MHz), which is output from the mixer 78.
To be more specific, if, for example, the tuning frequency of the main tuning circuit is 127 MHz, then the frequency of an oscillation signal will be 184 MHz, which is higher than the tuning frequency of the main tuning circuit by 57 MHz. Hence, the frequency 57 MHz is output from the mixer 78, the frequency 57 MHz representing the difference between 552 MHz, which is the frequency that is three times the frequency 184 MHz, and 609 MHz, which is the tuning frequency of the parasitic tuning circuits.
Referring back to FIG. 10, in order for the primary tuning circuit 51 and the secondary tuning circuit 52 to share the same magnitude of Q in the high-band receiving mode, equivalent current must be supplied to the switching diode 62 and the switching diode 80. This requires that the resistance values of the resistor 64 and the resistor 81 be the same. Furthermore, the DC resistance values of the switching diodes 62 and 80 must be reduced by passing current of 0.5 to 1.0 mA through the switching diodes 62 and 80. For this reason, the resistance values of the resistors 64 and 81 will be, for example, approximately 1 Kxcexa9, and it has been impossible to set any larger resistance values.
Referring now to FIG. 12 illustrating the low-band receiving mode, the resistors 64 and 81 that function to ground the cathodes of the switching diodes DC-wise when the diodes turn OFF are connected in parallel to the low-band receiving coils 56 and 73. As previously mentioned, small inter-terminal capacitance is present between the electrodes of the resistors 64 and 81, and the capacitance is connected in parallel to the varactor diodes 54 and 71, resulting in a reduced variable range of tuning frequency. Furthermore, the resistance values of the resistors 64 and 81 could not be increased more than, for example, approximately 1 Kxcexa9. Hence, the resistors 64 and 81 serve as parallel dampers for the tuning circuits 51 and 52, resulting in a lower gain.
FIG. 9 is a graph demonstrating the gain characteristics of a tuner employing a high-frequency tuning circuit in accordance with the present invention and a tuner employing a conventional high-frequency tuning circuit. Solid line A indicates the characteristic of a conventional example observed when the resistance values of both the resistors 64 and 81 are set to 1 Kxcexa9. As can be seen from solid line A of FIG. 9, the general gain drops, the gain of channel B of the maximum frequency in the low-band range showing a significant drop.
The present invention has been made with a view toward solving the problems described above, and it is a first object of the present invention to provide a double-tuned circuit of a tuner that exhibits a good selectivity that reduces the influences of parasitic tuning circuits 87 and 88 additionally constructed by inter-terminal capacitors 83 and 84 when the switching diodes 62 and 80 are OFF in the low-band receiving mode and the inter-terminal capacitors 85 and 86 of the resistors 64 and 81.
It is a second object of the present invention to provide a double-tuned circuit that exhibits a good selectivity in which damping relative to a tuning circuit in the low-band receiving mode is lessened, and to provide a double-tuned circuit that has a wider tuning range.
To these ends, according to one aspect of the present invention, there is provided a double-tuned circuit of a tuner, including: a primary tuning circuit; a secondary tuning circuit; and first and second band selecting terminals for applying a changeover voltage for switching the primary tuning circuit and the secondary tuning circuit to a low-band television signal receiving mode or a high-band television signal receiving mode, wherein each of the primary tuning circuit and the secondary tuning circuit has a varactor diode, a high-band receiving coil and a low-band receiving coil connected in series at each end thereof, and a switching diode connected high-frequency-wise between the connection point of the high-band receiving coil and the low-band receiving coil and ground; the varactor diode is connected in parallel to the high-band receiving coil and the low-band receiving coil connected in series, and the other end of the low-band receiving coil is grounded high-frequency-wise; one end of each switching diode grounded high-frequency-wise is connected to the first band selecting terminal DC-wise; the other end of the switching diode of one of the primary tuning circuit or the secondary tuning circuit is connected to the second band selecting terminal via a first resistor; and the other end of the switching diode of the other tuning circuit is connected to the second band selecting terminal DC-wise via the low-band receiving coil.
In a preferred form of the double-tuned circuit of a tuner in accordance with the present invention, the other end of the switching diode of the other tuning circuit is connected to the second band selecting terminal via the low-band receiving coil and the first resistor in series.
According to another aspect of the present invention, there is provided a double-tuned circuit of a tuner, including: a primary tuning circuit; a secondary tuning circuit; and first and second band selecting terminals, wherein each of the primary tuning circuit and the secondary tuning circuit has a varactor diode, a high-band receiving coil and a low-band receiving coil connected in series at their one end, and a switching diode that is provided between the connection point of the high-band receiving coil and the low-band receiving coil and the ground and has one end thereof grounded high-frequency-wise; the varactor diode is connected in parallel to the high-band receiving coil and the low-band receiving coil connected in series in each of the primary tuning circuit and the secondary tuning circuit, and the other end of the low-band receiving coil is grounded high-frequency-wise; one of each of the switching diodes is connected DC-wise to the first band selecting terminal and grounded via a second resistor, and the other end thereof is connected the second band selecting terminal via the low-band receiving coil and a third resistor in series; and the second band selecting terminal is grounded via a fourth resistor.
According to yet another aspect of the present invention, there is provided a double-tuned circuit of a tuner, including: a primary tuning circuit; a secondary tuning circuit; and first and second band selecting terminals, wherein each of the primary tuning circuit and the secondary tuning circuit has a varactor diode, a high-band receiving coil and a low-band receiving coil connected in series at their one end, and a switching diode that is provided between the connection point of the high-band receiving coil and the low-band receiving coil and the ground and has one end thereof grounded high-frequency-wise; the varactor diode is connected in parallel to the high-band receiving coil and the low-band receiving coil connected in series in each of the primary tuning circuit and the secondary tuning circuit, and the other end of the low-band receiving coil is grounded high-frequency-wise; the other end of each low-band receiving coil is isolated DC-wise; one of each of the switching diodes is connected to the first band selecting terminal DC-wise and grounded via a fifth resistor, and the other end thereof is connected the second band selecting terminal via a sixth resistor; and the other end of the switching diode of either the primary tuning circuit or the secondary tuning circuit is connected DC-wise to the second band selecting terminal via the low-band receiving coil and a seventh resistor.